<!DOCTYPE html>
<html lang="en">
  <head>
      <meta charset="UTF-8">
      <title>边框</title>
      <style type="text/css">
        div{
          /*width: 300px; 具体的像素*/
          width: 50%;
          height: 100px;
          border: 1px solid blue;
        }
      </style>
  </head>

  <body>
    <div>
      hello world~
    </div>
  </body>
</html>